This is a systematic long-term research program to develop mild chemical methods for peptide synthesis. The theoretical basis of these studies is the concept of orthogonal protection, and the experimental cornerstones are the new Na-dithiasuccinoyl (Dts) amino protecting group and the related class of open-chain carbamoyl disulfides of primary and secondary amines. These protecting groups can be rapidly and quantitatively removed under neutral conditions by reduction with thiols and other agents. A complete set of orthogonally protected Na-Dts amino acid derivatives are being prepared and applied to the synthesis of biologically active peptides. As a culmination of this line of research, the first total synthesis of an iron-sulfur protein, namely crystalline ferredoxin (55 residues) shall be attempted. Amino acid replacement analogues of this protein are contemplated which may shed light on the molecular details of electron transport. The principles of orthogonal solid-phase peptide synthesis are being further elaborated by the development of new anchoring linkages and new thiolysable side-chain protecting groups. The mild and specific chemistry of thiolyses of dithiasuccinoyl-amines and carbamoyl disulfides will be exploited for the directed synthesis of unsymmetrical disulfides and for the functional differentiation of Alpha-and Epsilon-amino groups of peptides and proteins. Also, methodology is being worked out to synthesize trisulfide-bridged analogues of disulfide-containing peptides. Advances in these areas will facilitate the preparation of fragile and structurally complex biomolecules, and clarify some of the important features necessary for their biological activities.